upgrade vs. research

i had a thought and if anyone knows the awnser please share, but if the research center gives boosts to all units, does the boost apply to whatever level it is or just when you do the boost? ie unit lvl 1 has 100 hp gains 100 hp per level so maxes out at 600. so if you research at lvl 1 he gains 3% or 3hp so if you research at lvl 1 and it adds 3 does it stay a boost of 3 or does it vary when you do it. the end result could be huge it could mean the difference between 603 hp and 618 hp for example.

From what i'm seeing it applies the percentage to what ever the current hp of the unit is. So it would be the 618 part of your example.